The One-electron Reduction of Carbonium Ions. VII. A Kinetic Study of the Reduction of the Aryl- and <I>n</I>-Alkyl-substituted Tropylium Ions with Cr(II)
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A series of substituted phenyltropylium (I) and <I>n</I>-alkyltropylium ions (II) has been synthesized, and the kinetic measurements for the one-electron reduction of these ions have been carried out with Cr(II) in 10% HCl at 25°C. A linear correlation is observed between the log<I>k</I><SUB>2</SUB> values, for I (X-C<SUB>6</SUB>H<SUB>4</SUB>-C<SUB>7</SUB>H<SUB>6</SUB><SUP>+</SUP>, X=<I>p</I>-OH, <I>p</I>-OCH<SUB>3</SUB>, <I>p</I>-CH<SUB>3</SUB>, H, <I>p</I>-Cl, <I>p</I>-Br, <I>p</I>-CN, <I>m</I>-OCH<SUB>3</SUB>) and for II (C<I><SUB>n</SUB></I>H<SUB>2<I>n</I>+1</SUB>-C<SUB>7</SUB>H<SUB>6</SUB><SUP>+</SUP>, <I>n</I>=0,1–6), and the transition energy of the charge-transfer band of these ions, with pyrene as a donor. In addition, the values of log<I>k</I><SUB>2</SUB> for I exhibit a good linear free-energy relationship with p<I>K</I><SUB><I>R</I><SUP>+</SUP></SUB> and also with the σ -value (ρ=+1.31), but not with σ<SUP>+</SUP>. The <I>n</I>-alkyl group is shown to diminish the reducibility of the tropylium ion, and a slight alternation effect of the length of the <I>n</I>-alkyl chain on the <I>K</I><SUB>2</SUB>'s is observed.
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